The recovery of hydrocarbons, such as oil and gas, from a subterranean well formation can be impeded by scales obstructing the flow of hydrocarbons from hydrocarbon-bearing zones of the formation. Typical scales are barite (e.g. BaSO4) or calcite (e.g. CaCO3) and it is common practice to treat these by bull-heading an aqueous-based scale dissolver fluid through a well bore and into the formation.
For example, one conventional scale dissolver for barite scale consists of a concentrated solution of potassium carbonate, potassium hydroxide and the potassium salt of ethylenediaminetetraacetic acid(EDTA), the corrosive and chelating nature of the solution being effective in removing scale. Carbonate scales may be dissolved using simple mineral acids, such as HCl.
However, hydrocarbon-producing wells often contain zones that are watered-out, producing only, or very largely, water. If the scale dissolver enters these zones, scale may also be removed therefrom. This can lead to an undesirable increase in the water cut of the fluid produced by the well.
In related but different fields of hydrocarbon recovery (notably the field of hydraulic fracturing, as described for example in EP-A-0835983), significant use is made of viscoelastic fluids. These fluids are typically based on aqueous solutions of surfactants, such as erucyl bis(2-hydroxyethyl) methyl ammonium chloride or potassium oleate, which can form worm-like micelles when mixed with brines, e.g. KCl brine. The structure of the micelles contributes significantly to the viscoelasticity of the fluid, and viscoelasticity is rapidly lost when the fluid contacts hydrocarbons which cause the micelles to change structure or disband.